1. Field of the Invention
This invention relates to dual priority camera, and more particularly to improvements in the accuracy of exposure control in both of the shutter priority and diaphragm priority modes.
2. Description of the Prior Art
In the dual priority cameras of which the light metering mode is performed at the full open aperture, it is generally required that, when in the diaphragm preselection automatic exposure control mode, an electrical signal representative of the set value of diaphragm aperture be formed to be applied to a shutter time computing circuit for computation of an effective shutter time, while when in the shutter preselection automatic exposure control mode, the position of the driven lens aperture mechanism be scanned to form an electrical signal also which is compared with an output signal of an automatic diaphragm control circuit to adjust the lens aperture mechanism to the proper presetting.
To achieve this, according to the prior art as disclosed in German Laid-Open Patent Application No. 2,415,615, the means for producing the aforesaid two signals are constructed in the form of a single variable resistor arranged to operate in the shutter preselection as well as the diaphragm preselection automatic exposure control modes.
With this single variable resistor operating in the shutter preselection automatic exposure control mode, it is the commonest practice that the shutter release leads to actuation of a diaphragm scanning mechanism with production of ever-varying electrical output signals which are compared with a signal representative of the proper diaphragm aperture value computed, and, when the difference between the magnitudes of these two signals compared has reached a predetermined level, an arresting mechanism is actuated to stop the scanning mechanism from further movement. On the other hand, when in the diaphragm preselection automatic exposure control mode, there is no possibility of creating a time lag in transmitting the output signal of that variable resistor depending upon the adjusted position of the diaphragm ring to the shutter time computing circuit.
As the inertia of the combined lens aperture and scanning mechanisms is rapidly increased with increase in the speed of movement thereof, the diaphragm position responsive signals occurring when in the shutter preselection and diaphragm preselection automatic exposure control modes are differentiated from each other. In other words, the time interval from the moment at which the difference in magnitude between the compared two signals has reached the predetermined level and the moment at which the movement of the lens aperture mechanism driven by the scanning mechanism gets stopped amounts up to a number of milliseconds by which the lens aperture mechanism is overrun to impair the accuracy of automatic diaphragm control.
In order to compensate for such error, with the camera of the single priority, that is, shutter priority type, provision may be made to shift the level by that error. But, with the camera equipped not only with the shutter priority but also with the diaphragm priority, it is made impossible by the level shift to obtain an electrical signal with magnitude exactly proportional to the manually set value of diaphragm aperture.
On this account, in the case of the dual priority camera, whether or not the level shift is electrically effected may be made dependent on selection of the shutter preselection and diaphragm preselection automatic exposure ranges. Even with this, it is impossible to achieve the establishment of the level shift near the start of operation of the diaphragm scanning means, because, in order to arrest the diaphragm scanning means near the start of scanning operation, the control circuit has to produce an actuating signal for the scanning means before the arresting means for the scanning means is actuated.
In the conventional dual priority camera, therefore, the speed of movement of the diaphragm scanning means is controlled so as to minimize the amount of error due to the slow responsibility of the arresting means and the inertia of the lens aperture and scanning means. The residual error is, however, not so much small as desired, and the decrease in the speed of automatic adjustment of the diaphragm aperture value will result in failure of snap photography.
The present invention has for its general object to provide a dual priority camera which has overcome the above-mentioned conventional drawbacks.
To achieve this, the signal representative of the set value of diaphragm aperture and the signal representative of the scanned value of diaphragm aperture are formed by respective separate means operating independently of each other, so that the error of diaphragm control which would be otherwise encountered when the common signal forming means is rendered operative in the shutter preselection automatic exposure range can be prevented from occurring.
Other objects and features of the invention will become apparent from the following detailed description of a specific embodiment thereof taken in conjunction with the accompanying drawings in which: